set/archive_sfl/2 matches the site

public static Set Physical_Hierarchy()
{
    var set = new Set()
        .WithContext()
        .WithId("https://linked.art/example/set/archive_sfl/2")
        .WithLabel("Stieglitz Family Letters")
        .WithClassifiedAs (Aat.ArchivalSubGrouping);

    set.IdentifiedBy = [ new Name("Stieglitz Family Letters").AsPrimaryName() ];

    set.MembersContainedBy = [
        new HumanMadeObject()
            .WithId("https://linked.art/example/object/box55")
            .WithLabel("Archival Box 55")
    ];

    return set;
}

• Role shortcuts AsPrimaryName, AsAccessionNumber and AsSystemAssignedNumber each add a single classification, with the AAT terms Primary Name, Accession Number and System Number.

var set = new Set
{
    Context = ["https://linked.art/ns/v1/linked-art.json"],
    Id = "https://linked.art/example/set/archive_sfl/2",
    Label = "Stieglitz Family Letters",
    ClassifiedAs = [Aat.ArchivalSubGrouping],
    IdentifiedBy =
    [
        new Name("Stieglitz Family Letters")
        {
            ClassifiedAs = [Aat.PrimaryName],
        },
    ],
    MembersContainedBy =
    [
        new HumanMadeObject
        {
            Id = "https://linked.art/example/object/box55",
            Label = "Archival Box 55",
        },
    ],
};

Generated from the object the example builds: every property that is set, as object initialisers, with the vocabulary constants where a term is in the registry. The fluent helpers are a convenience, not a requirement; this form and the one as written produce the same JSON.